The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device for use in a 1.3 xcexcm or 1.5 xcexcm wavelength band.
Optical wavelength band of 1.3 xcexcm or 1.5 xcexcm is used commonly in optical telecommunication systems that use optical fibers. It should be noted that a quartz glass optical fiber has an optical transmission band in the wavelength of 1.3 xcexcm or 1.5 xcexcm.
In correspondence to the foregoing specific optical transmission band of the optical fibers, conventional optical telecommunication systems generally use a laser diode constructed on an InP substrate. Such a laser diode typically uses an active layer of InGaPAs having a lattice constant matching the lattice constant of the InP substrate and a bandgap corresponding to the optical wavelength of 1.3 xcexcm or 1.5 xcexcm.
While the foregoing laser diode that uses InGaAsP active layer performs well in conventional optical telecommunication systems, particularly optical telecommunication trunks, the laser diode, requiring an expensive temperature regulation system such as a Peltier cooling device for a proper operation thereof, is deemed to be inappropriate for optical subscriber systems such as optical home terminals because of the increased cost of the temperature regulation system. In the foregoing laser diode that uses the InGaPAs active layer in combination with the InP substrate, the discontinuity of conduction band at the interface between the active layer and the surrounding cladding layer or optical waveguide layer is not sufficient for effective confinement of the carriers in the active layer, and there is a tendency that the carriers escape or overflow from the active layer when the device is not properly cooled. Because of such a poor confinement of the carriers, the laser diode generally shows a poor efficiency of laser oscillation. This problem becomes particularly serious in a high temperature operation of the laser diode where the carriers experience extensive thermal excitation.
On the other hand, recent investigations on a GaAs-GaN system have discovered that the bandgap of a GaAs mixed crystal containing therein a small amount of N decreases with increasing N content in the GaAs mixed crystal. GaN itself has been known to have a very large bandgap and is used for an active layer of an LED or laser diode that emits a blue or violet optical radiation.
FIG.1 shows the bandgap of such a GaAs-GaN mixed crystal system together with other group III-V compound semiconductor materials (Kondow, M., et al., Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995, pp. 1016-1018).
Referring to FIG. 1, it should be noted that, while GaN or a mixed crystal thereof containing a small amount of As has a very large bandgap suitable for emission of blue or violet optical radiation, the mixed crystal of GaAs containing a small amount of N has a small bandgap suitable for emission of the 1.3 xcexcm or 1.5 xcexcm optical wavelength band used for optical telecommunications systems. It should be noted that the bandgap of the GaAs mixed crystal decreases rapidly with increasing N content therein. Further, FIG. 1 indicates that the lattice constant of the GaAs mixed crystal decreases substantially with increasing N content therein.
Thus, the Japanese Laid-Open Patent Publication 6-334168 describes a technology of growing a III-V mixed crystal film containing N on a Si substrate epitaxially. For example, the foregoing reference describes a laser diode and a photodiode that use a GaNP cladding layer having a composition of GaN0.03P0.97 in combination with an active layer having a strained superlattice structure in which a GaNP layer and a GaNAs layer are stacked alternately and repeatedly. The foregoing cladding layer successfully establishes a lattice matching with the Si substrate. According to the teaching of the foregoing reference, it becomes possible to form a III-V device on a Si substrate without inducing misfit dislocations in the epitaxial layers. Further, the disclosed technology enables formation of an integrated circuit in which the III-V optical semiconductor devices are integrated monolithically with other Si devices.
Further, various mixed crystal compositions that establish a lattice matching with a substrate of GaAs, InP or GaP are reported for various N-containing III-V systems such as GaInNAs, AlGaNAs and GaNAs in the Japanese Laid-Open Patent Publications 6-037355.
Conventionally, no III-V composition has been known that has a bandgap smaller than the bandgap of GaAs and simultaneously a lattice constant that matches the lattice constant of GaAs, until a mixed crystal of GaInNAs is discovered. Provided that the N content is held small, the GaInNAs mixed crystal successfully establishes a lattice matching with a GaAs substrate and simultaneously has a bandgap smaller than the bandgap of GaAs. See the band diagram of FIG. 1. Thus, the GaInNAs mixed crystal is thought to be a promising material for an active layer of an optical semiconductor device that operates in the 1.3 xcexcm or 1.5 xcexcm wavelength band. However, little is known about the properties of the mixed crystal of GaInNAs.
Thus, Kondow, M., et al., op. cit., proposes a laser diode structure that uses a GaInNAs mixed crystal for the active layer of a laser diode. The reference further discloses the use of a cladding layer of AlGaAs in contact with the active layer of GaInNAs for securing a large discontinuity in the conduction band at the heterojunction interface across the cladding layer and the active layer. Because of the very large band discontinuity at the heterojunction interface, the laser diode is expected to show a high efficiency of laser oscillation and improved temperature characteristic associated with an efficient confinement of the carriers in the active layer.
On the other hand, It is known that the epitaxial growth of a GaInNAs mixed crystal is substantially difficult at high temperatures because of the tendency of the N atoms escaping from the deposited epitaxial layer of GaInNAs In order to obtain a film containing a substantial amount of N atoms, it is necessary to carry out the deposition process at a temperature of about 680xc2x0 C. or less. However, the epitaxial growth at such a low temperature is not preferable for the growth of a layer containing Al, such as a layer of AlGaAs used for the cladding layer, because of the tendency of the highly reactive Al atoms in the cladding layer reacting with a small amount of O atoms remaining in the deposition chamber or in the source gases as impurity. The O atoms thus incorporated form a non-optical recombination center in the epitaxial layer, while the non-optical recombination centers thus formed tend to annihilate the carriers without emitting photons. It should be noted that the problem of oxidation of Al cannot be avoided-even when the deposition is carried out under an environment where the air is purged by a high-performance vacuum system.
In order to avoid the foregoing problem of incorporation of the O atoms into the cladding layer of AlGaAs, it is necessary to carry out the deposition of the cladding layer at a high temperature of at least 750xc2x0 C. However, the use of such a high temperature growth is contradictory to the requirement of low temperature growth of the GaInNAs active layer as noted previously. Even in the case in which the substrate temperature is lowered after the deposition of the cladding layer of AlGaAs for allowing the deposition of the GaInNAs layer thereon, such a lowering of the substrate temperature also allows unwanted incorporation of the O atoms in the reaction chamber to the exposed surface of the lower cladding layer, and the formation of non-optical recombination centers at the heteroepitaxial interface between the lower cladding layer and the active layer is inevitable. It should be noted that the non-optical recombination centers reduce the lifetime of the optical semiconductor device such as a laser diode.
In addition to the foregoing problem, the inventor of the present invention has discovered that a direct epitaxial growth of a GaInNAs layer on an AlGaAs layer is difficult, as in the case of forming a laser diode that uses a cladding layer of AlGaAs in combination with the GaInNAs active layer.
FIG. 2 shows the surface morphology of a GaInNAs layer grown directly on an epitaxial layer of AlGaAs, which in turn is grown on a GaAs substrate. The GaInNAs layer is grown with a thickness of 0.1 xcexcm and has a composition of Ga0.9In0.1N0.03As0.97 for a successful lattice matching to the GaAs substrate. The AlGaAs layer underlying the GaInNAs layer has a composition of Al0.04Ga0.6As and is grown with a thickness of about 0.2 xcexcm.
Referring to FIG. 2, it will be noted that the surface of the GaInNAs layer is not smooth but includes minute projections and depressions, indicating a non-uniform or island-like growth of the GaInNAs layer occurring on the surface of the AlGaAs layer. The GaInNAs layer having such an irregular surface morphology performs poorly when used for the active layer of a laser diode due to various reasons such as scattering of light at the irregular surface or non-optical recombination of the carriers caused by the defects that accompany with such irregular heteroepitaxial interface.
Thus, it has been difficult to fabricate a double hetero laser diode that uses a III-V layer containing N for the active layer.
Accordingly, it is a general object of the present invention to provide a novel and useful optical semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide an optical semiconductor device operable in a 1.3 xcexcm or 1.5 xcexcm optical wavelength band without a temperature regulation.
Another object of the present invention is to provide an optical semiconductor device that includes an active layer containing N atoms therein, wherein a large band discontinuity is secured between the active layer and a cladding layer while reducing non-optical recombination centers simultaneously.
Another object of the present invention is to provide an optical semiconductor device, comprising:
a substrate;
a lower cladding layer substantially free from Al and provided on said substrate;
an active layer of GaInNPAs provided on said lower cladding layer; and
an upper cladding layer substantially free from Al and provided on said active layer.
According to the present invention, the upper and lower cladding layers are free from Al. Thus, the oxidation of Al in the upper or lower cladding layer at the time of low-temperature deposition of the active layer is successfully avoided. by using a mixed crystal of GaInNPAs for the active layer, it becomes possible to achieve a photo-electronic interaction in the active layer for an optical radiation having a 1.3 xcexcm or 1.5 xcexcm band.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer of a semiconductor material having said first conductivity type and provided on said substrate, said lower cladding layer having a composition substantially free from Al;
an active layer of a group III-V compound semiconductor material provided on said lower cladding layer, said active layer containing Ga and In as a group III element and N and As as a group V element;
an upper cladding layer of a semiconductor material having a second, opposite conductivity type and provided on said active layer, said upper cladding layer having a composition substantially free from Al;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower cladding layers are free from Al. Thus, the oxidation of Al in the upper or lower cladding layer at the time of low-temperature deposition of the active layer is successfully avoided. By using the group III-V semiconductor material containing Ga, In, N and As in combination with the GaAs substrate, the laser diode oscillates successfully in the 1.3 xcexcm wavelength band with high efficiency. As the carriers are confined effectively in the active layer in the laser diode of the present invention, the laser diode is operable without external cooling. By setting the composition of the active layer in lattice matching to the GaAs substrate, the active layer can be formed with a desired thickness, without inducing a lattice misfit strain therein. Thereby, the laser diode can have a double-hetero structure in which the active layer having a thickness suitable for acting as an optical waveguide is sandwiched directly by the cladding layers.
Another object of the present invention is to provide laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer of AlGaAs having said first conductivity type and provided on said substrate without accumulating a substantial lattice misfit strain;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer of a GaInNAs provided on said lower optical waveguide layer, said active layer being substantially free from a lattice misfit strain;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer of AlGaAs doped to a second, opposite conductivity type and provided on said upper optical waveguide layer without accumulating a substantial lattice misfit strain;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By using GaInPAs for the optical waveguide layers sandwiching the active layer, the carriers injected to the laser diode are confined effectively in the active layer as compared with the case of using GaAs for the optical waveguide layers due to the increased bandgap energy of GaInPAs that achieves a lattice matching to the GaAs substrate.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer of AlGaAs having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer of a GaInNAs provided on said lower optical waveguide layer, said active layer having a bandgap energy corresponding to a 1.3 xcexcm optical wavelength;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer of AlGaAs having a second, opposite conductivity type and provided on said upper optical waveguide layer;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of the low-temperature deposition of the active layer is successfully avoided. By using AlGaAs for the upper and lower optical waveguide layers, the thermal resistance of the cladding layer is reduced substantially, and the high-temperature stability of the laser diode operation is improved substantially. By using a mixed crystal of GaInNAs that contains N for the active layer of the laser diode, a laser oscillation in the 1.3 xcexcm band is successfully achieved while simultaneously achieving a lattice matching between the active layer and the GaAs substrate. By increasing the In content in the active layer, the bandgap of the mixed crystal forming the active layer is reduced and the laser oscillation wavelength is increased for the mixed crystal composition of the active layer in which the N content is reduced. By reducing the N content of the active layer as such, the quality of the crystal forming the active layer is improved, and the efficiency of laser oscillation and the laser oscillation spectrum are improved substantially.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer of AlGaInP having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer of a GaInNAs provided on said lower optical waveguide layer, said active layer having a bandgap energy corresponding to a 1.3 xcexcm optical wavelength;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer of AlGaInP doped to a second, opposite conductivity type and provided on said upper optical waveguide layer;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By using a mixed crystal of GaInNAs that contains N for the active layer of the laser diode, a laser oscillation in the 1.3 xcexcm band is successfully achieved while simultaneously achieving a lattice matching between the active layer and the GaAs substrate. By using GaInPAs for the upper and lower optical waveguide layers, a large band discontinuity is secured at the interface between the optical waveguide layer and the active layer, and the carriers injected to the laser diode are confined into the active layer with high efficiency. Thereby, the laser diode shows an excellent high temperature performance.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer provided on said lower optical waveguide layer, said active layer comprising an alternate repetition of a quantum well layer of GaInNAs and a barrier layer of GaInPAs;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer doped to a second, opposite conductivity type and provided on said upper optical waveguide layer;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate;
each of said quantum well layers accumulating therein a compressional lattice misfit strain and each of said barrier layers accumulating therein a tensile lattice misfit strain.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By forming a strained superlattice structure in the active layer by the quantum well layers of GaInNAs and the barrier layers of GaInPAs, it is possible to reduce the bandgap energy and increase the oscillation wavelength of the laser diode while simultaneously reducing the N content in the quantum well layers. Thereby, the quality of the crystal forming the quantum well layers is improved substantially and an efficient laser oscillation is achieved with a sharply defined oscillation spectrum.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer provided on said lower optical waveguide layer, said active layer comprising an alternate repetition of a quantum well layer of GaInNAs and a barrier layer of GaInNPAs;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer doped to a second, opposite conductivity type and provided on said upper optical waveguide layer;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
said quantum well layers accumulating therein a compressional lattice misfit strain and said barrier layers accumulating therein a tensile lattice misfit strain.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By forming a strained superlattice structure in the active layer by the quantum well layers and the barrier layers, it is possible to reduce the bandgap energy and increase the oscillation wavelength of the laser diode while simultaneously reducing the N content in the quantum well layers. Thereby, the quality of the crystal forming the quantum well layers is improved substantially and an efficient laser oscillation is achieved with a sharply defined oscillation spectrum. Further, the laser diode of the present invention is easy to fabricate, as the quantum well layers and the barrier layers are formed in the same deposition apparatus by merely switching the supply of a source of P on and off repeatedly.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of GaAs having a first conductivity type;
a lower cladding layer of GaInP having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer of GaInNPAs having a bandgap energy in a 0.8 xcexcm band and provided on said lower optical waveguide layer;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer of GaInP having a second, opposite conductivity type and provided on said upper optical waveguide layer;
a contact layer provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By using a mixed crystal of GaInNPAs for the active layer, the laser diode oscillates successfully at an optical wavelength of about 0.81 xcexcm. As a very large band-discontinuity is secured in the laser diode of the present invention that uses GaInP for the cladding layer, the laser diode has an excellent high temperature performance and is suitable for high power applications.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of InP having a first conductivity type;
a lower cladding layer of InP having said first conductivity type and provided on said substrate;
an active layer of a GaInNPAs provided on said lower cladding layer;
an upper cladding layer of InP having a second, opposite conductivity type and provided on said active layer;
a contact layer of a group III-V compound semiconductor material having said second conductivity type and provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the upper and lower cladding layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By using a mixed crystal of GaInNPAs for the active layer, the laser diode oscillates successfully at an optical wavelength band of about 1.5 xcexcm. As the carriers are confined effectively in the active layer in the laser diode of the present invention, the laser diode is operable without external cooling. By setting the composition of the active layer in lattice matching to the InP substrate, the active layer can be formed with a desired thickness, without inducing a lattice misfit strain therein. Thereby, the laser diode can have a double-hetero structure in which the active layer having a thickness suitable for acting as an optical waveguide is sandwiched directly by the cladding layers.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of InP having a first conductivity type;
a lower cladding layer of InP having said first conductivity type and provided on said substrate;
a lower optical waveguide layer of GaInPAs provided on said lower cladding layer;
an active layer provided on said lower optical waveguide layer, said active layer comprising an alternate repetition of a quantum well layer of GaInNPAs and a barrier layer of GaInPAs;
an upper optical waveguide layer of GaInPAs provided on said active layer;
an upper cladding layer of InP having a second, opposite conductivity type and provided on said upper optical waveguide layer;
a cap layer provided on said upper cladding layer;
a first ohmic electrode provided in ohmic contact to said cap layer; and
a second ohmic electrode provided in ohmic contact to said substrate.
According to the present invention, the upper and lower optical waveguide layers directly in contact with the active layer are free from Al. Thus, the oxidation of Al in the upper or lower optical waveguide layers at the time of low-temperature deposition of the active layer is successfully avoided. By using a mixed crystal of GaInNPAs for the active layer, the laser diode oscillates successfully at an optical wavelength band of about 1.5 xcexcm. As the carriers are confined effectively in the active layer in the laser diode of the present invention as a result of the use of InP cladding in combination with the GaInNPAs active layer, the laser diode is operable at high temperatures without external cooling.
Another object of the present invention is to provide a laser diode, comprising:
a substrate having a first conductivity type;
a lower cladding layer having said first conductivity type and provided on said substrate;
an active layer of a group III-V compound semiconductor material containing Ga and In as a group III element and N and As as a group V element;
an upper cladding layer having a second, opposite conductivity type and provided on said active layer;
a current confinement structure provided on said upper cladding layer, said current confinement structures including first and second patterns disposed on said upper cladding layer at both lateral sides of an optical axis of said laser diode when viewed perpendicularly to said substrate so as to expose a part of said upper cladding layer along said optical axis, each of said first and second patterns being formed of a group III-V compound semiconductor material containing Ga and In as a group III element and N and As as a group V element, said first and second patterns having said first conductivity type and a bandgap energy not exceeding a bandgap energy of said active layer;
a second upper cladding layer of said second conductivity type provided on said current confinement structure so as to cover said first and second patterns forming said current confinement structure and in contact with said exposed part of said upper cladding layer;
a contact layer of said second conductivity type provided on said second upper cladding layer;
a first ohmic electrode provided in ohmic contact with said contact layer; and
a second ohmic electrode provided in ohmic contact with said substrate.
According to the present invention, the first and second patterns, formed of a mixed crystal containing Ga, In, N and As, effectively absorb the optical radiation produced in the active layer. Thereby, the refractive index of the first and second patterns is changed and an optical waveguide structure confining an optical radiation laterally along the optical axis of the laser diode is formed along the axial direction of the laser diode. In other words, the optical radiation produced in the active layer is confined effectively in the lateral direction, and the stimulated emission of photons is substantially facilitated as the optical beam is guided by the optical waveguide structure thus formed back and forth in the axial direction of the laser diode. As the first and second patterns are formed to have the first conductivity type, the confinement of the carriers occurs also to the exposed part of the upper cladding layer that contacts with the second upper cladding layer. It is preferred to form the first and second patterns of the same material forming the active layer.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.